1

PUBLIC SWITCHED TELEPHONE NETWORKS PSTN

Synchronous Transfer Mode (STM)Time-Division-Multiplexing (TDM)Circuit switching -RoutingRouting: Connection Oriented

Networking Key word

Asynchronous Transfer Mode (ATM)Statistical Multiplexing (SM)Packets Switching Routing:Routing: Connection/Connectionless

Oriented

Time Division Multiplexing

SCHEDULER

T1 T2 Tm

BROADBAND BUS

Multiplexing with scheduling

•Assume that we have m communication terminals, T1, T2, .., Tm sharing a transmission line, how do we schedule the sharing of communication bandwidth?

• Assume that the bandwidth is shared by the terminals transmitting at different times.

• We also assume that a scheduling mechanism is available so that the transmissions are conflict free, namely, that no two terminals attempt to transmit at the same time.

• We call this scheduled or arbitrated access communication.

• In the absence of an arbitration mechanism, two communication terminals may transmit at the same time, often resulting in unintelligible transmissions.

Two basic approaches to multiplexing:

1. The first approach assumes a common time reference among the terminals. We call this time reference a frame reference. The communication bandwidth assigned for each terminal is termed a circuit. This mode of multiplexing is commonly knownas the Synchronous Transfer Mode (STM).

2. The second approach assumes no frame reference among the terminals, hence the name Asynchronous Transfer Mode (ATM). This mode allows more flexible sharing of bandwidth by avoiding rigid bandwidth assignments.

Bandwidth is seized on demand, and the information transmitted (together with a proper label) upon a successful seizure is termed a packet.

The Asynchronous Transfer Mode• The definition of a frame depends on the bit-rates of the terminals

multiplexed on the transmission link.

• The choice of frame structure is difficult since we have littleknowledge of the traffic mix.

• An alternative approach abandons the concept of a frame reference altogether. Instead of choosing a basic terminal bit-rate as in TDM, ATM achieves more flexible bandwidth sharing allowing the terminals to seize bandwidth when a sufficient number of bits are generated.

• Without a frame reference, these bits have no implicit ownership, unlike STM for which each slot is assigned an owner.

• Hence a key feature of ATM is that information from each terminal must be labeled.

2

The Asynchronous Transfer Mode

There are many forms of asynchronous multiplexing:

• First, we may have fixed length blocks of information from eachterminal.

•These blocks are termed cells in ATM terminology.

• A cell is labeled block of transmitted information, and usually has a small information payload (typically from 32 bytes to 128 bytes).

• We shall also refer to them as short fixed length packets.

The Asynchronous Transfer Mode

Cell (or Short fixed length packets):

• Each cell or packet has a fixed size of l bits. The channel is slotted into fixed intervals of duration l/C, each transporting a cell.

• The terminals are asynchronous in the sense that they have no common time reference other than the common slot reference.

• A label for each time slot must be provided by the terminal which transmits in that time slot.

The Asynchronous Transfer Mode

The label identifies the terminal generating the bits delivered in the time slot. A label is included in the header part of a packet. The header may serve other functions; such as classifying the information payload (type and priority), and possible error check sums for protecting the header from transmission error.

t

l BITS SLOTS

INFO

HEADER

PACKET

Multiplexing of Fixed Length Packets

The Asynchronous Transfer Mode

There are two major factors in determining the proper packet size:

First, headers use up part of the communication capacity of the link. This overhead is inversely proportional to the packet size l, consequently favoring long packet.

Second, a packetization delay is needed for the terminal to collect the l bits for a packet. The delay between signal generation and reception is given by , t = l/bplus the delay taken for the signal to travel in the network.

For some applications, excessive delay results in perceivable degradation of the quality of communication.

Consequently, minimizing packetization delay requires choosing short packets. A compromise has to be chosen between two opposing factors.

The Asynchronous Transfer Mode

Variable Length Packets:

Instead of short fixed length packets, it is often convenient (particularly for data communications) to use long (say 128 bytes or more) variable length packets.

Besides the label for ownership, the packet header should also contain the information for packet length to mark the end of thepacket, as well as a flag to mark the beginning of the packet.

t

l BITS SLOTS

INFO

HEADER

PACKET

Multiplexing of Fixed Length Packets

3

Local Exchange Carriers (LECs)♦ LECs provide local telephone service, usually within the boundaries of a

metropolitan area, state, or province.♦ LECs also provide short-haul, long distance service, Centrex, certain enhanced

services such as voice mail, and various data services.♦ BOCS (Bell Operating Companies), originally were wholly owned by AT&T,

dominated the ILECs landscape.

Local Access and Transport Area (LATA)♦ Effective January 1, 1984, those 22 BOCs were spun off from AT&T as a result of

the Modified Final Judgement (MFJ).♦ BOCs were reorganized into seven Regional BOCS (RBOCS).♦ BOCs were limited to providing basic voice and data services within defined

geographical areas, known as Local Access and Transport Areas (LATAs).

♦ Are some 170 areas defined by the MFJ♦ Collectively span all BOC territories♦ In general, each Boc territory comprises several LATAs

PSTN PSTN ContinueInterExchange Carriers (IXCs or IECs) ♦ IXCs are responsible for long-haul, long-distance connections across LATA boundaries.♦ IXC networks are connected to the LECs through a Point of Presence (POP) which typically is in the form of a tandem switch.♦ A POP is a location where IXC interfaces BOC for exchange access to IXC services.♦ The IXC POP is connected to the LEC access tandem switch via dedicated trunks leased from the LEC. Alternatively, the IXC may collocate network termination equipment in the LEC office, assuming that space is available and that secure physical separation can be established and maintained.♦ IXCs provide inter-lata services.

Basic Architecture of a PSTN

Central end

office

Remote Terminal

(RT)

Central Tandem office

LEC Domain

POP

Tandem Switch

Tandem Switch

Tandem Switch

Access (Local) Network

Feeder Network

Distribution Network

Regional Network Long-distance Network

IXC Domain

Switch

POPCustomer

PBX

Direct Access Switched AccessSwitch

POP

LEC End

Office

Customers

Switch

POPLEC End

Office

LEC Access

Tandem

Customers

Customer has large enough volume of traffic accessing the POP or requiring egress from it to pay for the direct connect facility, bypassing the LEC switching network.

Customer traffic to/from POP doesn’t justify direct connect.

• The IXC purchase access/egress facilities from the LEC which uses its switched network to deliver/receive that traffic.

IXC Access Types

Office Park

CAP Fiber Ring

Switch

CAP

ATT POP

Sprint POP

MCI POP

IXC domainEnd user access to an IXC via a CAP, bypassing the LEC

Achieving Connectivity

Full Mesh Shared Medium

4

Role of SwitchingConnectivity, network resource sharing, customer coordination

Sharing Transmission Bandwidth

Dedicated Line

Time Shared Synchronous

TDM

Time Shared Packet, Burst

Circuit Switching Circuit” refers to the capability of transmitting one telephone conversation along one link. To set up a call, a set of circuits has to be connected, Joining the two telephone sets. By modifying the connections, the operators can switch the circuits. Circuit switching occurs at the beginning of a new telephone call. Operators were later replaced by mechanical switches and, eventually, by electronic switches. An electronic interface in the switch converts the analog signal traveling on the link from the telephone set to the switch into a digital signal, called a bit stream. The same interface converts the digital signal that travels between the switches into an analog signal before sending it from the switch to the telephone. The switches use a dedicated data communication network “Common channel signaling (CCS)” to exchange control information among themselves. Thus CCS separates the functions of call control from the transfer of voice.

Circuit Switching Continue ♦ In current telephone networks, the bit streams in the trunks (lines

connecting switches) and access links (lines connecting subscriber telephones the switch) are organized in the digital signal (DS) hierarchy.

♦ The DS-1 signal carries 24 DS-0 channels, but its rate is more than 24

times 64 kb/s. The additional bits are used to accommodate DS-0 channels with rates that deviate from the nominal 64 because the signals are generated using clocks that are not perfectly synchronized.

♦ Since the 1980s the transmission links of the telephone network have

been changing to the SONET or Synchronous Optical Network, standard.

♦ In circuit switching, the route and bandwidth allocated to the stream

remain constant over the lifetime of the stream.

CCiirrccuuiitt SSwwiittcchhiinngg CCoonnttiinnuuee ♦♦ TThhee ccaappaacciittyy ooff eeaacchh cchhaannnneell iiss ddiivviiddeedd iinnttoo aa nnuummbbeerr ooff ffiixxeedd--rraattee llooggiiccaall

cchhaannnneellss,, ccaalllleedd cciirrccuuiittss.. TThhee ddiivviissiioonn iiss uussuuaallllyy aaccccoommpplliisshheedd bbyy TTDDMM.. CCiirrccuuiitt sswwiittcchhiinngg iinnvvoollvveess tthhrreeee pphhaasseess:: ((11)) TThhee ssoouurrccee mmaakkeess aa ccoonnnneeccttiioonn oorr ccaallll rreeqquueesstt ttoo tthhee nneettwwoorrkk,, tthhee nneettwwoorrkk

aassssiiggnnss aa rroouuttee aanndd oonnee iiddllee cciirrccuuiitt ffrroomm eeaacchh lliinnkk aalloonngg tthhee rroouuttee,, aanndd tthhee ccaallll iiss tthheenn ssaaiidd ttoo bbee aaddmmiitttteedd ((iiff tthhee nneettwwoorrkk iiss uunnaabbllee ttoo mmaakkee tthhiiss aassssiiggnnmmeenntt,, tthhee ccaallll iiss rreejjeecctteedd)).. TThhiiss pphhaassee iiss ccaalllleedd ccoonnnneeccttiioonn sseettuupp..

((22)) DDaattaa ttrraannssffeerr nnooww ooccccuurrss--tthhee dduurraattiioonn ooff tthhee ttrraannssffeerr iiss ccaalllleedd tthhee ccaallll

hhoollddiinngg ttiimmee.. ((33)) WWhheenn tthhee ttrraannssffeerr iiss ccoommpplleettee,, tthhee rroouuttee aanndd tthhee cciirrccuuiittss aarree ddeeaallllooccaatteedd..

TThhaatt pphhaassee iiss ccaalllleedd ccoonnnneeccttiioonn tteeaarrddoowwnn..

Rate in Mb/s Meium Signal No. of Voice

Circuits North America Europe

T-1 paired Cable

DS-1 24 1.5 2.0

T-1C paired cable

DS-1C 48 3.1

T-2 paired cable

DS-2 96 6.3 8.4

T-3 coax, radio, fiber

DS-3 672 45.0 32.0

Coax, waveguide, radio, fiber

DS-4 4032 274.0

Digital Signal Hierarchy

Note that the bit rate of a DS-1 signal is greater than 24 times the rate of voice signal (64 Kb/s) because of the additional framing bit required.

5

Circuits / Time Slots

•• TTDDMM iiss iiddeeaall ffoorr ccoonnssttaanntt bbiitt rraattee ttrraaffffiicc.. •• TThhee ccaappaacciittyy ooff tthhee oouuttggooiinngg cchhaannnneell iiss ddiivviiddeedd iinnttoo NN llooggiiccaall cchhaannnneellss.. •• TTiimmee oonn tthhee oouuttggooiinngg cchhaannnneell iiss ddiivviiddeedd iinnttoo ffiixxeedd--lleennggtthh iinntteerrvvaallss ccaalllleedd ffrraammeess.. •• FFrraammeess aarree ddeelliimmiitteedd bbyy aa ssppeecciiaall bbiitt sseeqquueennccee ccaalllleedd aa ffrraammiinngg ppaatttteerrnn.. •• TTiimmee iinn eeaacchh ffrraammee iiss ffuurrtthheerr ssuubbddiivviiddeedd iinnttoo NN ffiixxeedd--lleennggtthh iinntteerrvvaallss ccaalllleedd

sslloottss//cciirrccuuiittss.. •• EEaacchh ffrraammee ccoonnssiissttss ooff aa sseeqquueennccee ooff sslloottss:: sslloott 11,, sslloott 22,,....,, sslloott NN.. ((AA sslloott iiss uussuuaallllyy

11 bbiitt oorr 11 bbyyttee wwiiddee)).. •• AA llooggiiccaall cchhaannnneell ooccccuuppiieess eevveerryy NNtthh sslloott.. TThheerree aarree tthhuuss NN llooggiiccaall cchhaannnneellss.. TThhee

ffiirrsstt llooggiiccaall cchhaannnneell ooccccuuppiieess sslloottss 11,, NN ++ 11,, 22NN ++ 11,,....;; tthhee sseeccoonndd ooccccuuppiieess sslloottss 22,, NN++22,, 22NN++22,,......;; aanndd ssoo oonn..

Time Division Multiplexing

...

... ...

Channel 1

Channel 2

Channel N

1 12 2N N

Frame 1 Frame 2

Synchronous Transfer Mode

PBX

Workstation

Router

STM

Multiplexer

STM Multiplexing is also known as Time Division Multiplexing (TDM)

13 23 12

• The T1 Frame (or the OSI term, PDU) consists of 24 8-bits slots.

The TDM multiplexer operates as follows:• The data bits in each incoming channe1 are read into a separate FIFO (first in,

first out) buffer.

• The multiplexer reads this buffer in sequence for an amount of time equal tothe corresponding slot time: buffer 1 is read into slot 1, buffer 2 is read into slot2, etc.

• If there are not enough bits in a buffer, the corresponding slot remains partiallyempty.

• The bit stream of the outgoing channel is easily demultiplexed: thedemultiplexer detects the framing pattern from which it determines the begi-nning of each frame, and then each slot.

TDM Continues

...

Channel 1

Channel 2

Channel N

1 N 21

Statistical Multiplexing (SM)

♦Most effective in the case of bursty input data.

♦As in TDM, the data bits in each incoming channel are read into separate FIFOs.

♦The multiplexer reads each buffer in turn until the buffer empties.

♦The data read in one turn is called a “data packet”.

Asynchronous Transfer Mode

Workstation

PBX

Router

ATM

Multiplexer

C

B

A

Z

Y

YZ Y Z Z Z

SM Continues ♦ In TDM each FIFO is read for a fixed amount of time-one slot-and

so each incoming channel is allocated a fixed fraction of the outgoing channel capacity, independent of the data rate on that channel.

♦ By contrast, in SM, the capacity allocated to each incoming

channel varies with time, depending on the instantaneous data rate: the higher the rate, the larger the capacity allocated to it at that time.

♦ The size of packets read from each FIFO can vary across channels

and over time within each channel. ♦ The demultiplexer cannot sort the packets belonging to different

channels merely from their positions within a frame.

6

SM Continues ♦ Additional bits, which delimit each packet and identify the corresponding

incoming channel or source, must be added to each packet. ♦ The resulting overhead is significantly larger than under TDM. ♦ Multiplexer and demultiplexer implementations are more difficult; ♦ Multiplexer must now add the packet delimiter and channel or source

identifier. ♦ Demultiplexer must locate and decode those bit patterns. ♦ These increases in complexity and overhead must be balanced against high utilization in the face of bursty data to determine whether SM or TDM is more efficient.

DATA COMMUNICATIONSDATA COMMUNICATIONS

Data BBiinnaarryy CCooddeess♦♦ BBeettwweeeenn mmaacchhiinneess,, iinnffoorrmmaattiioonn iiss eexxcchhaannggeedd bbyy bbiinnaarryy ddiiggiittss ((bbiittss))..

TTwwoo sseettss aarree iinn ccoommmmoonn uussee ttooddaayy::AASSCCIIII:: tthhee AAmmeerriiccaann SSttaannddaarrdd CCooddee ffoorr IInnffoorrmmaattiioonn IInntteerrcchhaannggeeeemmppllooyyss aa sseeqquueennccee ooff sseevveenn bbiittss.. SSiinnccee eeaacchh bbiitt mmaayy bbee 00 oorr 11,, AASSCCIIIIccoonnttaaiinnss 112288 uunniiqquuee ppaatttteerrnnss..

EEBBCCDDIICC:: tthhee EExxtteennddeedd BBiinnaarryy CCooddeedd DDeecciimmaall IInntteerrcchhaannggee CCooddee eemmppllooyyss aa sseeqquueennccee ooff eeiigghhtt bbiittss.. IItt ccoonnttaaiinnss 225566 uunniiqquuee ppaatttteerrnnss..

♦♦ TThheerree aarree ttwwoo bbaassiicc mmeetthhooddss ooff ddaattaa ttrraannssmmiissssiioonn AAssyynncchhrroonnoouuss aannddSSyynncchhrroonnoouuss..

AAssyynncchhrroonnoouuss ((CChhaarraacctteerr FFrraammeedd)) TTrraannssmmiissssiioonn;;♦♦ CChhaarraacctteerrss aarree ggeenneerraatteedd aanndd ttrraannssmmiitttteedd ssiinnggllyy,, oonnee aafftteerr tthhee ootthheerr..

♦♦ IInn ssoommee tteerrmmiinnaallss,, tthhee cchhaarraacctteerrss aarree ccoolllleecctteedd uunnttiill aa ccoommpplleettee lliinnee oofftteexxtt iiss ccrreeaatteedd,, oorr tthhee rreettuurrnn kkeeyy iiss pprreesssseedd,, ccaauussiinngg tthhee lliinnee ttoo bbee sseenntt aass aabbuurrsstt ooff ccoonnttiinnuuoouuss cchhaarraacctteerrss..

Data Continues

♦ WWhheetthheerr sseenntt oonnee--bbyy--oonnee aass tthheeyy aarree ggeenneerraatteedd,, oorr sseenntt lliinnee--bbyy--lliinnee aass eeaacchh lliinneeiiss ccoommpplleetteedd,, eeaacchh cchhaarraacctteerr iiss ffrraammeedd bbyy aa ssttaarrtt bbiitt ((00)) aanndd aa ssttoopp bbiitt ((11))

SSyynncchhrroonnoouuss ((MMeessssaaggee FFrraammeedd )) TTrraannssmmiissssiioonn::♦ Such transmission is message framed and overcome the inefficiencies of asynchronous, start-stop transmission for high speed data transmission.

♦ Rather than surrounding each character with start and stop bits, arelatively large set data is framed, or blocked with one or moresynchronization bits or bit patterns used to synchronize the receivingterminal on the rate of transmission of the data.

♦ The start sequence is called the header – it contains synchronizing,address, and control information. The stop sequence is called the trailer –it contains error checking and terminating information.

♦ The entire data entity is called a “Frame”

Stop Bit (1) Start Bit (0)

Character

Framed characters sent as they are created -- a data stream typical of keyboard input to a terminal or

communications controller.

Framed characters that are concatenated and sent when a Framed characters that are concatenated and sent when a string is completed string is completed ---- a datastream typical of a terminal a datastream typical of a terminal

sending keyboard input linesending keyboard input line--byby--line to a communications line to a communications controllercontroller

Trailer HeaderCharacterFrame

Data Block

Asynchronous Transm

ission Format

In asynchronous transmission, each character is framed by one start bit and one or two stop bits.

Characters are assembled Characters are assembled into a datablock that is into a datablock that is framed by a header and a framed by a header and a trailer to produce a frame. trailer to produce a frame. The frame is sent when a The frame is sent when a command is received from command is received from the controlling unit in the the controlling unit in the communication system.communication system.

Synchronous Transmission Format

Sender ReceiverMessage Message

Datastream that includes redundant bits and the result of the sender’s calculations

Sender adds redundant bits and performs calculations to assist the

receiver in error detection

Receiver checks redundant bits and repeats calculations looking for agreement with sender’s results

Because each character is assigned a unique code, it is extremely important to be sent without error. For instance, the ASCII code for p is 11100001110000. An error in bit # 1produces 1110001110001 which is the code for q.

◆ Error detection is a cooperative activity between the sender and the receiver in which a sender adds information to the character or frame to assist the receiver in determining whether an error has occurred in transmission or reception.

Error Control/DetectionError Control/Detection

7

Sender performs calculation...

MK

Gn+1= integer + Fn

Receiver performs same calculation...MK

Gn+1= integer + F’n

If F’n = Fn’ transmission is without errorIf F’n ≠ Fn’ transmission is without error

Sender adds Frame Check

Sequence (Fn) to frame

Receiver re-calculates

Fn

Cyclic Redundancy Check

MK MKMKFn

Gn+1Gn+1

Generating Function

Generating Function

Error CorrectionError Correction

Once detected,an error must be corrected. Two basic approaches to error correction:1. Automatic-Repeat-Request (ARQ):Requires the transmitter to re-send the portions of the exchange in which errors have been detected. ARQ techniques include: •Stop-and-Wait: The sender sends a frame and waits for acknowledgement from the receiver. This technique is slow.•Go-back-n:

2. Forward Error Correction (FEC): FEC techniques employ special codes that allow the receiver to detect and correct a limited number of errors without referring to the transmitter. This convenience is bought at the expense of adding more bits (more overhead)

DTE

DTE

EIA232 DCE

EIA232 DSU/CSU

Analog (Voice Grade) Line

Data Circuit Terminating Equipment

Digital Signals

MODEM

Data Terminal Equipment

Digital Line

• The data equivalent of Customer Premise Equipment (CPE) in the voice world, Data Terminal Equipment (DTE) comprises the computertransmit and receive equipment; are digital devices that send or receivedata messages.

• Internally, their signals are simple, unipolar pulses; externally, they may use one the more sophisticated digital signaling schemes.

Data Communication

♦ Data Circuit Terminating Equipment (DCE): is the equipment that interfaces the DTEto the network; maps the incoming bits into signals appropriate for the channel, and atthe receiving end, maps the signals back to bits.

♦ DCEs includes mmooddeemmss, digital service units ((DDSSUUss)),, and channel service units((CCSSUUss)).

♦ If the transmission channel is an analog line (voice-grade), the DCE is called amodem. When sending, DCE convert the ddiiggiittaall ssiiggnnaall received by the DTE toaannaalloogg ssiiggnnaallss to match the bandwidth of the channel.

♦ If the connections are digital connections, the DCE consists of two parts: DDSSUU-- receives uunniippoollaarr ddiiggiittaall ssiiggnnaallss from the DTE and converts them to bbiippoollaarr ssiiggnnaallss. CCSSUU: provides loopback (for testing), limited diagnostic capabilities. When sending, it converts bipolar signals to AMI.

Data Communication Continues

EEIIAA223322 iinntteerrffaaccee♦ A DET is connected to a DCE by a cable that conforms to EIA232 standard.♦ EIA232 describes a multi-wire cable that terminates in 25-pin connectors.♦ The cable supports asynchronous or synchronous operation at speed up to

19.2 kb/s. At 19.2 kb/s, the cable length is limited to 50 feet.♦ The EIA232 circuits linking DTE and DCE carry signals that initiate,

maintain, and terminate communication between the two.

HHiigghheerr SSppeeeedd IInntteerrccoonnnneeccttiioonnss EEIIAA444499:: It permits operation up to 2 Mb/s at distances up to 4000 feet.

EEnntteerrpprriissee SSyysstteemmss CCoonnnneeccttiioonn ((EESSCCOONN)):: an optical fiber connection operating up to 40 kilometers at 17 Mb/s.

FFiibbeerr CChhaannnneell SSttaannddaarrdd ((FFCCSS)):: Operates up to 10 kilometers at speeds up to 800 Mb/s. FCS includes error control and switching.

ProtocolsProtocolsData Link Control (DLC) Protocol→ A set of rules that governs the exchange of messages over a data link.DLC protocols are divided into two classes:• Asynchronous Operation:→ Start-Stop DLC protocol• synchronous Operation:→ Bit-oriented DLC protocol (e.g., SDLC): Introduced in 1972, SDLC

was modified and standardized by ITU-T and ISO as:⇒ HDLC (High Level Data Link Control Protocol)⇒ LAP-B (Link Access-Procedure Balanced), for X.25 Standard⇒ LAP-D ((Link Access-Procedure Channel), for ISDN-D Channel⇒ LAP-F ((Link Access-Procedure Frame Relay), a version of LAP-D

used in Frame Relay applications.

Different in the detailed meaning of specific control field bits, all of these protocols share a common structure. In the order that they are transmitted, they consist of the following fields: FlagFlag, AddressAddress, ControControl, TextText, Frame Check SequenceFrame Check Sequence, and FlagFlag.

8

Start Bit

Line Idle State

0 11 0 0 0 0 1

Timing Mark

CHARACTER ASCII ‘a’

1

Stop Bit

Line Idle State

Time between characters

10 1 0 0 0 10

Start Bit

Stop Bit

1

CHARACTER ASCII ‘b’

Line Idle State

Timing Mark

Transmission Format for StartTransmission Format for Start-- stop (Asynchronous) stop (Asynchronous) Signaling. In idle state, the line is maintained at the 1 Signaling. In idle state, the line is maintained at the 1 level. The start bit (0) reduces the level to zero level. The start bit (0) reduces the level to zero signaling the commencement of activity.signaling the commencement of activity.

FLAG

Address

FLAGC

ontr

ol FCS

TEXT

usually 1024 bits(not Supervisory Frames)

Header Trailer

SDLC FRAME

8 bits

24 8 816N x 8

0111111001111110

0 FNS NRInformation Format

1 PMode NRSupervisory Format

0NO TEXT

NR Receive Sequence Number

Number (in sequence 000

through 110⇒) of frame

expected. 111

acknowledges sequence of seven frames.

NS Send Sequence Number

Number (in sequence 000

through 110⇒) of this

frame.

Mode 00 = Ready to Receive

10 = Not ready to Receive

01 = Reject

P = 0 = not polled

1 = poll

F = 0 = more frames to come. Information transfer is not

complete.

1 = last Frame

SDLC Frame Format

PACKET SWITCHING

Packet Switching ♦ The data stream originating at the source is divided into packets of fixed or

variable size. ♦ The time interval between consecutive packets may vary, depending on the

burstiness of the stream. ♦ As the bits in a packet arrive at a switch or router; they are read into a

buffer when the entire packet is stored, the switch routes the packet over one of its outgoing links.

♦ The packet remains queued in its buffer until the outgoing link becomes

idle. This store-and-forward technique thereby introduces a random queuing delay at each link;

♦ The delay depends on the other traffic sharing the same link. Packets from

different sources sharing the same link are statistically multiplexed.

Packet Switching Continues In datagram packet networks, each packet within a stream is independently routed. ♦ A routing table stored in the router (switch) specifies the outgoing link for each

destination. The table may be static, or it may be periodically updated. ♦ Each packet must contain bits denoting the address of the source and destination. In virtual circuit packet networks, a fixed route is selected before any data is transmittedin a call setup phase similar to circuit-switched networks. ♦ However; there is no notion of a fixed-rate circuit or logical channel. All packets

belonging to the same data stream follow this fixed route, called a virtual circuit. ♦ Packets must now contain a virtual circuit identifier; this bit string is usually shorter than

the source and destination address identifiers needed for datagrams. However; the callsetup phase takes time and creates a delay not present in datagram packet networks.

The routing decision

Connectionless (datagra Connectionless (datagram) Connection Oriented (virtual circuit)

Connection-Oriented vs Connectionless Transport

Could changeMaintainedMaintainedPacket Sequence

“Share Pain”“Share Pain”“Busy”Overload

SharedSharedGuaranteedBandwidth

VariableVariableConstantDelay

NoYesYesConnection State

Shared

Resource

Guaranteed Resource

Connectionless

Connection Oriented

Circuits and Virtual Circuits

9

Connection Oriented Packet Transport

• Connection Request • Resource Check• Route Selection • Destination Acceptance• Connection begins

Connectionless Transport

• Lower Level Protocol (IP)“Send and Pray”

• Upper Level ProtocolGuaranteed delivery

Rel

ay

Tec

hniq

ues

Direct Connection

Store & forward

Hold & forward

Hold & forward

Hold & forward

Med

ia Copper, wireless

Copper, wireless

Copper, wireless, optical

Copper, wireless, optical

Copper, wireless, optical

Size

of

PDU No such

thing

Variable, large to small

Variable, large to small

Variable, large to small

Fixed, very small

Del

ay Very Fast Slow Fast Faster Very Fast

Circuit Switching

Message Switching

Packet Switching

Frame Relay (Switching)

Cell Relay (Switching)

Switching Technologies

Fast Relay

Frame Relay(Variable size PDU’s--frames)

Cell Relay(Fixed size PDU’s--cells)

PVC

(LAPD)

SVC

(Q.931)

802.6 Based

(For SMDS)

ATM Based

(For B-ISDN)

PVC SVC

(Q.2931)

Types of relay systems

UserUser

X.25

X.75 (NNI)

X.25

= Packet switches

Typical X.25 Topology

X.25 is not a packet switching specification. It’s a packet netwX.25 is not a packet switching specification. It’s a packet networkorkinterface specification. X.25 says nothing about operations withinterface specification. X.25 says nothing about operations within in the network. the network.

It Provides for an interface between an end-user device (DTE) and a network (DCE). Its formal title is “Interface between DTE and DCEfor terminals operating in the packet node on public data networks”

In X.25, the DCE is the “agent” for the packet network to the DTE.

X.25 ContinueX.25 Continue

X.25 encompasses the lower three layers of the OSI modelX.25 encompasses the lower three layers of the OSI model

X.25X.25--3 layer (network layer)3 layer (network layer)Packets are created at the network layer that Establishes, manage, and teardown the connections between the user and the network.

X.25X.25--2 layer (data link layer)2 layer (data link layer)The packet is encapsulated within the Link Access Procedure, Balanced(LAPB) protocol as the information field. The LAPB protocol is a sub-set of HDLC (High Level Data Link Control).

X.25X.25--1 layer (physical layer)1 layer (physical layer)The physical layer is the physical interface between the DTE and the DCE.

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X.25 Continue ♦ X.25 uses logical channel numbers (LCNs) to identify the DTE connections to the

network. An LCN is really nothing more than a virtual circuit identifier (VCI). ♦ Octets #1 and Octet #2 of the packet header provide a 12-bit identifier. If all-zeros

possibility is excluded, as many as 4095 logical channels (i.e., user sessions) can be assigned to a physical channel.

♦ The LCN serves as an identifier (a label) for each user's packets that are transmitted

through the physical circuit to and from the network. ♦ Typically, the virtual circuit is identified with two different LCNs-one for the user at the

local side of the network and one for the user at the remote side of the network. ♦ X.25 provides two mechanisms to establish and maintain communications between the

user devices and the network (and ATM has borrowed these concepts): Permanent Virtual Circuit (PVC) and Switched Virtual Circuit (SVC).

X.25 Continue ♦♦ PPVVCCss mmaayy ssuuppppoorrtt llaarrggee uusseerrss.. AAllll ppaacckkeettss ttrraavveell tthhee ssaammee ppaatthh bbeettwweeeenn ttwwoo ccoommppuutteerrss;;

wwhhiicchh ppaatthh iiss eessttaabblliisshheedd bbyy rroouuttiinngg iinnssttrruuccttiioonnss pprrooggrraammmmeedd iinn tthhee iinnvvoollvveedd nnooddeess.. ♦♦ TThhee cciirrccuuiittss iinnvvoollvveedd iinn tthhee rroouuttee aarree ddeeffiinneedd oonn aa ppeerrmmaanneenntt bbaassiiss,, uunnttiill ssuucchh ttiimmee aass

tthheeyy aarree ppeerrmmaanneennttllyy rreeddeeffiinneedd,, ppeerrhhaappss aass tthhee sseerrvviiccee ♦♦ AAlltteerrnnaattiivveellyy,, tthhee nneettwwoorrkk mmaayy sseelleecctt tthhee mmoosstt aavvaaiillaabbllee aanndd aapppprroopprriiaattee ppaatthh oonn aa ccaallll--

bbyy--ccaallll bbaassiiss uussiinngg SSwwiittcchheedd VViirrttuuaall CCiirrccuuiittss ((SSVVCCss));; ♦♦ AAggaaiinn,, aallll ppaacckkeettss iinn aa ggiivveenn sseessssiioonn ttrraavveell tthhee ssaammee ppaatthh.. ♦♦ SSVVCCss ddeemmaanndd aa ggrreeaatteerr lleevveell ooff nneettwwoorrkk iinntteelllliiggeennccee tthhaatt aaddddss ttoo ttoottaall nneettwwoorrkk ccoosstt;; tthhiiss

ttrraannssllaatteess iinnttoo hhiigghheerr ccoosstt ttoo tthhee eenndd--uusseerr oorrggaanniizzaattiioonn.. ♦♦ TThhee eessttaabblliisshhmmeenntt ooff aa SSVVCC aallssoo iinnvvoollvveess ssoommee lleevveell ooff ddeellaayy ssiinnccee tthhee nneettwwoorrkk nnooddeess

mmuusstt eexxaammiinnee mmuullttiippllee ppaatthhss iinn oorrddeerr ttoo mmaakkee aa pprrooppeerr sseelleeccttiioonn..

Transport

Packet X.25-3

LAPB X.25-2

X.21

X.25-1DTE

LAPB

X.21

Data Link

Physical

Network

Packet Header

PacketData

LAPB Header

LAPB Trailer

Data

DCE

User’s DataUser Stack

USER-NETWORK INTERFACE X.25

PACKET NETWORK

USER’S INFORMATIONI.e. message data and/or headers from upper layers

User’s Data Segment

FLAG

Address

Control

FCS

FLAG

Packet Headers

User’s Data Segment 1024

bits

1 D QLogical Grp #

Logical Channel Number

0 P(S) M P(R)

User’s Data Segment

User’s Data Segment

User’s Data Segment

PacketHeader Trailer

HDLC FRAME

X.25 Packet and Frame Format

0

COMPUTER NETWORKS

The RS-232-C standard for the serial line specifies the transfer of one 8-bit character at a time, separated by time intervals. The speed and distance of the serial line are limited.

RS-232-C (1969)

2.4 – 38 Kbps

01101011_11011010_

11

The Synchronous Data Link Control and related The Synchronous Data Link Control and related standards transmit long packets of bits. The header (H) standards transmit long packets of bits. The header (H) contains the preamble that starts the receiver clock, contains the preamble that starts the receiver clock, which is kept in phase by the selfwhich is kept in phase by the self-- synchronizing synchronizing encoding of the bits. The receiver uses the cyclic encoding of the bits. The receiver uses the cyclic redundancy check (CRC) bits to verify that the packets is redundancy check (CRC) bits to verify that the packets is correctly received.correctly received.

A B

CD

E

StoreStore--andand--forward transmissions proceed by sending the packetforward transmissions proceed by sending the packetsuccessively along links from the source to the destination. Thesuccessively along links from the source to the destination. Thepacket header specifies the source and destination addresses (A packet header specifies the source and destination addresses (A and E, for example) of the packet. When it receives a packet, a and E, for example) of the packet. When it receives a packet, a computer checks a routing table to find out on which link it computer checks a routing table to find out on which link it should next send the packet.should next send the packet.

Ethernet. In this network, computers are attached to a Ethernet. In this network, computers are attached to a common coaxial cable. The computers read every transmitted common coaxial cable. The computers read every transmitted packet and discard those not addressed to them.packet and discard those not addressed to them.

B

C

D

E

A A B

C DE

Token ring. The computers share a ring. Access is regulated Token ring. The computers share a ring. Access is regulated by a tokenby a token-- passing protocol.passing protocol.

4 or 16 Mbps

A B

C DE

Fiber Distributed Data Interface (FDDI). A tokenFiber Distributed Data Interface (FDDI). A token--passing passing protocol is used to share the ring. The computers time their protocol is used to share the ring. The computers time their holding of the token. This network guarantees that every holding of the token. This network guarantees that every computer gets to transmit within an agreedcomputer gets to transmit within an agreed--on time.on time.

100 Mbps

155-622 Mbps

A B

CD

E

Asynchronous Transfer Mode (ATM) network. The network Asynchronous Transfer Mode (ATM) network. The network transports information in 53transports information in 53--byte cells. Total throughput ofbyte cells. Total throughput ofthis network is much larger than that of FDDI or of a 100this network is much larger than that of FDDI or of a 100--Mbps Mbps Ethernet. Ethernet.

12

LAYERING APPROACHLAYERING APPROACH RAM

VR

AM

DISK

CPU

CA

CH

E

Display

NIC

Keyboard, mouse, etc.

Computer

CPU

RAM

NIC

User System

Message TransfersThe left panel gives a simple architecture of a host computer and its connection to the network. The right panel shows the four copiesthat may be involved across the CPU bus to run an application, reducing the host throughput.

OSI Hierarchy

• Physical– SONET, T1, T3

• Link– Ethernet, FDDI– Circuit, ATM, FR

switches• Network

– Routing, Call control– IP internetworking

Physical

Transport

Network

Link

Application

Presentation

Session

1

4

3

2

7

6

5

OSI Hierarchy

• Transport– Error and congestion

control– TCP, UDP

• Session, Presentation, Application– Data, voice encodings– Authentication– web/http, ftp, telnet

Physical

Transport

Network

Link

Application

Presentation

Session

1

4

3

2

7

6

5

Data Transfer Over Frame-based Networks

File

TCP

IP

Frame(Ethernet,FR, PPP)

Data Transfer Over Cell-based Networks

File

TCP

IP

Adaptation

ATM Cells

13

Internet Protocol Architecture

RTPRTP

LANsLANs PPPPPPATMATM FRFR

TCPTCP UDPUDP OSPFOSPF

BGPBGP

SNMPSNMPDNSDNSTELNETTELNETFTPFTP

SMTPSMTP

HTTPHTTPPingPing

ICMP

IP

RIPRIP

10/100BaseT10/100BaseT Dedicated B/W: DSx, SONET, ...

Dedicated B/W: DSx, SONET, ...

Circuit-Switched B/W: POTS, SDS, ISDN, ...

Circuit-Switched B/W: POTS, SDS, ISDN, ...

CDPDCDPD

WirelessWireless

Why a Synchronous Network“Visibility” of each byte at the line rate• Simplification of the multiplexing and

switching process

• Simple access to overhead bytes

“Stuffing” BitsOH OH

AsynchronousAsynchronous

SynchronousSynchronous

Overhead functions – framing, monitoring, fault location, protection switching, management communications.